Zimmerman definitely did initiate the contact, he was following Martin around the complex while talking to 911.
On the other hand, Trayvon Martin, a troubled teen who was drifting into the gangsta life, took Zimmerman's continued observation of him as justifying a sudden and severe attack on Zimmerman. Martin quickly put Zimmerman on the ground, pummelling his face MMA style, and slamming Zimmerman's head onto the concrete sidewalk. This put Zimmerman in fear of his life.
If Zimmerman had not been armed, he would have suffered a severe, potentially life-altering or lethal beating. In effect, Zimmerman did what many here insist the McMasters should have done, follow, observe, and call the cops.